[#boost-urls-grammar-lut_chars-2constructor-02]
= xref:boost.adoc[boost]::xref:boost/urls.adoc[urls]::xref:boost/urls/grammar.adoc[grammar]::xref:boost/urls/grammar/lut_chars.adoc[lut_chars]::lut_chars
:relfileprefix: ../../../../
:mrdocs:


Constructor.

== Synopsis


Declared in `<https://www.github.com/boostorg/url/blob/develop/include/boost/url/grammar/lut_chars.hpp#L227[boost/url/grammar/lut_chars.hpp]>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<class Pred>
requires detail::is_pred<Pred>::value &&
        ! std::is_base_of<
            lut_chars, Pred>::value
constexpr
lut_chars(Pred const& pred) noexcept;
----

== Description


This function constructs a character        set which has as members, every value        of `char ch` for which the expression        `pred(ch)` returns `true`.


=== Example

[,cpp]
----
struct is_digit
{
    constexpr bool
    operator()(char c ) const noexcept
    {
        return c >= '0' && c <= '9';
    }
};

constexpr lut_chars digits( is_digit{} );
----

=== Complexity

Linear in `pred`, or constant if        `pred(ch)` is a constant expression&period;


=== Exception Safety

Throws nothing&period;



== Parameters


|===
| Name | Description

| *pred*
| The function object to        use for determining membership in        the character set&period;


|===



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